In cellular mobile radio systems it is fundamental that a mobile station with an established connection on a radio channel shall be able to maintain the established connection when moving from one cell serviced by a base station to another cell serviced by another base station. It is also highly desirable that the mobile station with an established connection on a radio channel shall be able to maintain the established connection when moving within the same cell and when the radio channel which is used is subject to increased interference. The process by which a mobile station can maintain an established connection when moving in a cellular radio system is generally called a handoff.
In general, radio communication is only possible when the desired information-carrying radio signals have sufficient signal strength at the receiver and are sufficiently strong relative to the noise and interfering radio signals at the receiver. The minimum strength, of course, depends on the particular features of the system, e.g. the kind of modulation and the type of receiver. In order to ensure that the established connection may continue on an intended radio channel between a mobile station and an intended base station, the handoff process includes measurements of the parameters of the radio signals at the intended base station and/or at the mobile station.
The first cellular mobile systems placed in public use were analog systems typically used for speech or other types of analog information. These systems include multiple radio channels for transmitting analog information between base and mobile stations by transmitting analog modulated radio signals. In general, the first cellular mobile radio systems had relatively large cells, and the signal measurements in the handoff process in such systems were performed by the base station. One such system is the Nordic Mobile Telephone System NMT 450. Another known cellular mobile radio system is the AMPS Mobile Radio System in the United States. An excellent general description of a mobile cellular radio system can be found in a publication entitled "CMS 88 Cellular Mobile Telephone System" published by Ericsson Telecom AB, 1988. The rapidly increasing usage of these mobile radio systems often causes the cells to be utilized at maximum capacity.
A method of balancing the load among cells which are operating at maximum capacity is described in U.S. Pat. No. 4,670,899, by Brody, et al., and entitled "Load Balancing for Cellular Radio Telephone System". The aforementioned patent describes a load balancing technique for a cellular telephone system. The loading of the various cells is dynamically redistributed by selectively transferring ongoing calls to adjacent cells in accordance with traffic levels in order to reserve channels for handoffs and for new calls. A channel occupancy level for a cell is periodically determined by comparing the number of channels utilized to the number of channels available within the cell. Consequently, calls are handed off before all the channels are utilized, thereby allowing at least one or more channels to be reserved for new or incoming calls.
According to the Brody et al. patent, if there is a mobile unit on the periphery of the cell which is also within the range of a neighboring cell, the unit on the periphery will be ordered to transfer to the neighboring cell in order to make room for a new call or a mobile station entering the cell and engaged in an ongoing call. This procedures has two disadvantages. Firstly, it provides traffic based control of handoffs out of a cell, but there is no equivalent control of handoffs into a cell. Secondly, it creates a complex system, with handoffs due to load balancing being handled differently from handoffs due to mobile stations leaving the cell.
Another prior art system is disclosed in U.S. Pat. No. 4,435,840 by Kojima et al., and entitled "Radio Mobile Communication System Wherein Probability of Loss of Calls is Reduced Without a Surplus of Base Station Equipment." The aforementioned patent describes a mobile communication system in which the service area of a base station is narrowed by reducing the base station output power. Alternatively, the mobile station output power may be increased, or the base station reference level for call set-up may be varied. The disclosure of the Kojima et al. patent does not discuss the manner in which the handing off of established calls takes place.